Urchin-like CoP with Controlled Manganese ... - ACS Publications

Subscriber access provided by Washington University | Libraries

Article

Urchin-like CoP with controlled manganese doping toward efficient hydrogen evolution reaction in both acid and alkaline solution Yuancai Ge, Jiyi Chen, Hang Chu, Pei Dong, Steven R Craig, Pulickel M. Ajayan, Mingxin Ye, and Jianfeng Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Urchin-like CoP with controlled manganese doping toward efficient hydrogen evolution reaction in both acid and alkaline solution Yuancai Gea, Jiyi Chena, Hang Chua, Pei Dongb,c, Steven R Craigb, Pulickel M. Ajayanc, Mingxin Yea*, Jianfeng Shena* a

Institute of special materials and technology, Fudan University, 2200 Handan road, Yangu district, Shanghai 200433, China, [email protected], [email protected] b Department of Mechanical Engineering, George Mason University, 4453 Nguyen Engineering Building, Fairfax, Virginia 22030, USA c Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, USA

Abstract: Splitting water to produce hydrogen through an efficient and low-cost way requires the development of catalysts based on earth-abundant elements. Using d-band theory to modify the band structure, we verified Mn atoms to be potent dopants to reinforce the activity of urchin-like CoP greatly, and this catalyst could reach a current density of 10 mA/cm2 with overpotentials of only 65 mV and 100 mV in acid and alkaline, which were much superior to pristine CoP and close to Pt/C catalyst. Also, the outperformance of its durability was tested for 20 hours to maintain a current density of 10 mA/cm2. The increments of overpotentials were 2.4 mV and 1.1 mV in acid and alkaline, respectively. After introducing partial Mn atoms, the density functional theory calculation revealed that the Gibbs free energies of hydrogen adsorption (∆G

ads,H)

of Mn-doped CoP (-0.07 eV) was much smaller than

pristine CoP (-0.157 eV). Furthermore, density of state analysis indicated that the strong interaction between Co atoms and Mn atoms lead the d-band of Co atom to a negative shift, which weakens the forceful adsorption between the hydrogen and Mn-doped CoP and expedited the release of produced hydrogen efficiently.

Keywords: Hydrogen evolution reactions, Cobalt phosphide, Nano arrays.

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

Introduction The growing concern over global warming and environmental damage has triggered the urgent need for renewable energy carriers as an alternative to fossil fuel. Hydrogen, with zero-carbon footprint, is considered to be one of the most ideal replacements. And recent decades have witnessed the great progress of producing hydrogen from environmentally sustainable methods, such as electrocatalysis of water driven by solar energy and electrochemical water splitting with an overpotential.1-3 Pt, as the precious metal, is known to be the most active catalyst for hydrogen evolution reactions (HER).4 Nonetheless, the rare abundant and high cost of Pt has hampered its practical usage in a large scale. In this regard, much effort has been paid to the research for low-cost and abundant electrode toward efficient HER. Hitherto, among pyrite phase metal dichotomous such as FeS2 and CoSe2,5-6 two-dimensional metal dichotomous such as MoS2, MoSe2, WS2 and WSe2,7-9 and other metal carbide and nitride,10-14 the most effective catalysts were based on metal phosphide (NiP, Co2P, NiP2).15-16 In order to enhance the HER performance of different metal phosphide, various approaches have been developed including optimizing their crystal structures, morphologies and components.17-20 Generally, introducing different metal atoms to the metal phosphides has been an efficient strategy to improve their electrochemical performance. Among various metal phosphides, CoP was found to be the most typical catalyst with different doping elements.21-24 Sun’s group has demonstrated the success of introducing various elements, such as Fe, Zn, and Al, to the CoP.21,

23-24

After

introducing Fe to the CoP nanowires, the required overpotential decreased from 128 mV to 78mV to drive a geometry current density of 10 m A/cm2. Similarly, to reach a current density of 10 mA/cm2, adding partial Zn and Al to the CoP could lead to an overpotential decrease from 62 mV to 26 mV for Al-CoP, and from 99 mV to 67 mV for Zn-CoP. The work reported by Qu’s group suggested Ce to be an effective doping metal for CoP nanowires.22 To reach a current density of 10 mA/cm2, the applied overpotential of Ce-CoP was only 54 mV, which was much lower than the CoP


Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanowires (139 mV). In this work, we found that Mn, the second most abundant transition metal in the earth’s crust (~0.1%), could enhance the HER performance of urchin-like CoP significantly, when its ratio is controlled. The most efficient Mn-doped CoP in this work could reduce the values of overpotential from 118 mV to 65 mV to reach a current density of 10 mA/cm2 in 0.5M H2SO4. After durability test for 20 hours, a small increment of overpotential of about 2.4 mV and 1.1 mV was observed in alkaline and acid, respectively. Together with the systematic experiment, the density functional theory (DFT) method was applied to estimate the Gibbs free energies of hydrogen adsorption (∆G ads,H) on different sites, including the metal site, the phosphorus site, and the bridge site. The ∆G ads,H of cobalt site in Mn-doped CoP was only -0.07 eV, which was much lower than the pristine CoP (-0.157 eV) and other phosphorus sites. To further investigate the band structure of CoP, we applied the previously reported d-band theory,25 which demonstrated that the H adsorption energy was mainly effected by the d-orbital electron of the Co atoms. And Mn atoms with lower electron negative (1.55 versus 1.88 of Co) could diminish adsorption strength of hydrogen on the Co site significantly. The density of state (DOS) patterns of Co illustrated the negative shift of the d-band of Co atom after replacing partial Co atoms with Mn atoms, which could explain the efficient HER activity of Mn-doped CoP.26-27 The outstanding performance of Mn-doped CoP could be explained the following: (i) the urchin-like morphology of Mn-doped CoP could provide more exposed active site for HER.28-29 (ii) the band structure of Co atoms was adjusted via introducing partial Mn atoms, which lead to a more thermal-neutral value of ∆G ads,H. (iii) the binder-free interface between the metallic CoP and the titanium substrate could expedite the electron transferring more efficiently.30 Systematic experiments with detailed DFT analysis were carried to certified the enhanced HER activity of Mn-doped CoP in relative to pristine CoP. And the down shift of the d-band center from the Fermi level could diminish the binding energy between Mn-doped CoP and hydrogen atoms. Thus, we think previously reported d-band was a powerful guideline to predict and design various catalysts beyond HER catalysts.31 Besides, manganese ACS Paragon Plus Environment


was much cheaper than other dopant elements like Ce, Al and Zn but the performance of Mn doped CoP was comparable to other catalysts. Because the concentration of Mn in CoP was easily adjustable. Experimental Section Materials Cobalt chloride hexahydrate, manganese chloride tetrahydrate, urea, ammonium fluoride, sodium hypophosphiteand and hydrochloric acid were brought from Sinopharm Chemical. All chemical reagents were used without further purification. Titanium foil (0.1mm in thickness) was purchased from Sigma-Aldrich and sonicated in 3M HCl for one hour to remove the gray titanium oxide. Synthesis of Mn-doped cobalt precursors and cobalt precursor First, 4 mmol ammonium fluoride, 10 mmol urea and totally 2 mmol cobaltous chloride hexahydrate and manganese chloride tetrahydrate (with different ratio of manganese from 0%, 5%, 10%, 15% to 25%) were dissolved in 40 ml deionized water. After transferring the transparent solution to a 60 ml Teflon-lined stainless-steel autoclave, a piece of fresh titanium foil (10 mm×10 mm in size) was immersed into the solution. Then, the sealed autoclave was placed in an electric oven and maintain at 90 °C for 16 hours and then cooled down to room temperature. The titanium covered with Mn-doped cobalt precursor was washed with deionized water for three times and dried in a vacuum oven at 80 °C for 8 hours. Then, the titanium covered with Mn-Co precursors was heated to 450 °C for 2 hours under the nitrogen flow to obtain the MnxCo3-xO4 precursors. Synthesis of Mn-doped CoP and CoP In a 10 ml crucible, 30 mg sodium hypophosphite was placed in the bottle and the titanium foil covered with MnxCo3-xO4 was placed in the middle with a piece of


Page 4 of 25

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


carbon cloth as the separator. Then, the crucible was placed in a tube furnace and heated to 350 °C in 90 minutes and maintained at 350 °C for another 2 hours. Note that the as-prepared samples were named CoP, 5Mn-CoP, 10Mn-CoP, 15Mn-CoP and 25Mn-CoP due to the different ratio of Mn. The area mass loadings of different catalysts were calculated to be 376 µg/cm2, 373 µg/cm2, 379 µg/cm2, 381 µg/cm2, and 382 µg/cm2. We synthesized the Mn2P through the same method to synthesize CoP and the calculated area mass loading was 278 µg/cm2. Materials Characterization X-ray photoelectron spectroscopy (XPS) measurements were performed on a Perkin-Elmer PHI 5000C ESCA equipped with a Mg anode as the exciting source. X-ray diffraction (XRD) measurements were carried out on a Bruker D8 Advance X-ray diffractometer equipped Cu Kα radiation with a scan rate of 5 °/min. Field emission scanning electron microscopy (FESEM) measurements were made on Tescan MAIA3 XMH with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) images were collected on JEM-2100 with an acceleration voltage of 200 kV. Scanning transmission electron microscopy (STEM) mapping was conducted with an Oxford X-man 65T, and the TEM sample was prepared by sonicating 0.5 mg catalyst scratched from the titanium for 4 hours in 1 ml ethanol. The actual concentrations of Mn element were determined by using inductively coupled plasma atomic emission spectrometry (ICP-AES, Thermo Fisher I-Cap 7400). Electrochemistry measurement Electrochemical measurements were measured with an Autolab PGSTAT302N



Page 6 of 25

electrochemical workstation in a standard three-electrode system to measure the electrochemical activity and stability of the as-prepared samples. The titanium foil covered with our specimens, polished glass carbon electrode and saturated calomel electrode (SCE) were used as the working electrode, the counter electrode and the reference electrode, respectively. Adjusted via Nernst equation, all the acquired data was calibrated to reverse hydrogen electrode (RHE) by the equiation as below: E (RHE) = E (SCE) + 0.0591 pH + 0.241

(1)

All the potentials demonstrated in this work were versus RHE. Before each test, a nitrogen flow of 30 ml/min was used to degas the electrolyte for 5 min to eliminate the dissolved oxygen in the solution and the nitrogen flow was subsequently maintained during each test. The linear sweep voltammetry (LSV) measurements were recorded in both acid and alkaline solutions with a scan rate of 10 mV/s. The electrochemical impedance spectroscopy (EIS) measurements were achieved with a fixed of bias 150 mV from 1000 HZ to 0.01 HZ. The electrochemical surface area (ECSA) was proportional to double layer capacitance (Cdl), and various scan rates (from 25mV/s to 150 mV/s) of cyclic voltammetry (CV) were performed to calculate Cdl. DFT calculation All structures and DOS were calculated using density functional theory (DFT) via CASTEP code with an ultrasoft-pseudopotential, which was implemented in the Materials Studio package.32 The generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerh (PBE) functional was used to approximate the electron exchange-correlation potential.33-34 The semi-empirical dispersion correction of Grimme scheme was adopted, and a 4×4×2 k-point mesh was sampled based on


Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Monkhorst-Pack method. The applied cut-off energy was 600 eV. According to previous works, many calculations were based on the (101) surface of CoP which was considered as the most thermal stable one. Hence, in this work, all the calculated hydrogen adsorption energy was based on the (101) slabs of CoP and Mn-doped CoP, and at least four layers were adopted with the outmost layer allowed to relax. Periodical structures were optimized under Broyden–Fletcher–Goldfarb– Shanno schemes. The maximum force, maximum displacement, and convergences of energy were set at 0.01 eV/Å, 5.0×10-4 Å and 5.0×10-6 eV/atom. The adsorption energy of hydrogen atom in the faces of CoP was calculated as below. ∆E

ads, H=

Eslab+H – Eslab-EH2/2, where Eslab+H and Eslab were the total energy of slabs with and without hydrogen atom, respectively. The EH2 is the energy of H2 in the gas phase. According to previous works, the Gibbs free energy (∆Gads,H) was calculated from ∆E ads,H +

∆EZPE - T∆S H, where the ∆EZPE and T∆S H were attributed to zero-point energy

change and entropy difference of hydrogen from the absorbed state to the gas phase, respectively.34-36 Results and Discussion Composition and structure characterizations Figure 1 exhibits the morphology of the MnxCo3-xO4 precursors (Figure 1a) with urchin-like structures (about 3 µm in diameter). Increasing the amount of Mn didn’t affect their morphology, which could be explained, because every precursor shared the same crystal structure. After phosphidation at 350 °C for another 2 hours, the urchin-like morphology of Mn doped CoP was well inherited (Figure 1b). To further



illustrate the element dispersion and crystal structure of Mn-doped CoP, TEM measurements were carried out. Mn-doped CoP still preserved the urchin-like morphology (Figure 1c), and the HRTEM images demonstrate the lattice fringe of 0.29 nm and 0.25 nm were a bit larger than the interplanar distance of the (011) and (111) planes of CoP (Figure 1d). A similar result was also reported in Sun’s work via adding partial Al atoms to CoP.21 The element mapping image in Figure 1e exhibits the homogeneous dispersion of Co, P, and Mn. Also, the EDX analysis in Figure S1 verifies that the element ration of Co, Mn and P was about 1:0.21:1.2. The actual concentrations of Mn in differrnt Mn-doped CoP were further certified by ICP-AES measurement. As it was shown in Table S1, the actual concentrations of Mn in different samples were in agreement with their theoretical concentration. And the Mn concentration of 15Mn-CoP was 16.8 %.


Page 8 of 25

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. SEM images of cobalt precursor with different ratio of Mn(a), corresponding Mn doped CoP (b), TEM image of 15Mn-CoP (c), HRTEM image of 15Mn-CoP (d), and STEM mapping images(e)

Figure 2a depicts the detailed XRD patterns of pristine Co3O4 precursor together with the MnxCo3-xO4 precursors belonging to spinel structures. And in Figure 2b, it was clear to notice the negative shift of the (311) planes along with the increment of Mn owning to the larger atom radius of Mn than that of Co atom.37-38 The complete transformation of Mn-doped cobalt precursors to Mn-doped CoP was identified with XRD

patterns

in

Figure

2c.

The

diffraction

peaks

centered

at

31.6°,

32°,36.3°,46.2°,48.1° and 56.8° were indexed to the (011), (002), (111), (112), (211) and (301) planes of standard CoP phase (JCPDS No. 29-0497). No obvious peaks of other impurity and the negative shift of (011) planes were depicted in Figure 2d,



which has verified the efficient synthetic process of CoP and Mn-doped CoP.39 However, increasing the ratio of Mn to 25%, some peaks of Mn2P were observed in Figure S2, which demonstrated recognizable phase separation from Mn doped CoP to Mn2P. Figure 2e has elaborated the strategy of fabricating Mn-doped CoP with urchin structure.

Figure 2. XRD patterns of MnxCo3-xO4 precursors (a), enlarged XRD patterns of MnxCo3-xO4 precursors (b), XRD patterns of CoP and Mn-doped CoP (c), enlarged XRD patterns of CoP and Mn-doped CoP (d), schematic illustration of fabricating MnxCo3-xO4 precursors and Mn-doped CoP (e).


Page 10 of 25

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In addition, Figures S3 and S4 demonstrate the XPS spectrums of CoP and 15Mn-CoP. As shown in Figure S3 and Figure S4, the oxygen and carbon elements originated from the surface oxidation and the contamination of the XPS chamber. The peak centered at 133.5 eV derived from the partial oxidation of the phosphate samples, while the peaks centered at 130 eV and 129 eV were assigned to the 2p1/2 and 2p3/2 of P, respectively. The Co 2p3/2 region could be resolved into two main peaks centered at 778.1 eV and 781.9 eV with one satellite peak at 784.8.0 eV. The Co 2p1/2 region could also be resolved into two main peaks at 793.8 eV and 798.1 eV with one satellite peak at 802.4 eV. In Figure S4, the 2p3/2 region of Co has exhibited an increment of the peak centered at 781.9 eV. This area ratio for 15Mn-CoP (61%) was slightly larger than the value of the pristine CoP (52%), which implied the decrement of Co3+/Co2+ ratio after adding Mn atoms.22 The chemical state of Mn in 15Mn-CoP was identified by XPS. And the high-resolution 2p region of Mn demonstrated the peaks centered at 643 eV and 645.5 eV were ascribed to the Mn2+ and Mn3+, respectively.40-41 The observed chemical state of Mn3+ indicated the partial surface oxidation during the synthetic process. The replacement of Co ions with Mn ions would donate partial electrons to the neighbor Co ions and produce additional Co2+. Thus, the observed decrement of Co3+/Co2+ ratio certified the success of doping CoP with Mn.

Electrochemical activity and stability Electrochemistry activities of CoP and different Mn-doped CoP were tested in both acid and alkaline. Figure 3a illustrated the LSV curves of the obtained samples in alkaline. Pt/C was highly active for HER which required a overpotential of -60 mV to drive a current density of 10 mA/cm2 with a small Tafel slope of 39 mV/dec. For the pristine CoP, to reach a current density of 10 mA/cm2, the applied overpotential was 180 mV in 1M KOH. When increasing the amount of Mn in CoP, the HER activities were enhanced remarkably, and 15Mn-CoP acted as the most efficient catalyst,



needing an overpotential of 100 mV to drive a current density of 10 mA/cm2. However, when the ratio of Mn was augmented to 25%, a small reduction in HER activity was also observed. This could be ascribed to the emerging of the Mn2P from the Mn doped CoP phase as depicted in Figure S2. The Tafel slopes for CoP, 5Mn-CoP, 10Mn-CoP, 15Mn-CoP, and 25Mn-CoP were 73 mV/dec, 72 mV/dec, 67 mV/dec, 53 mV/dec and 66 mV/dec, respectively, implying all HER occurred via a Volmer-Heyrovsky mechanism (Figure 3b). The straightforward comparison among the catalysts is illustrated in Figure 3c, and the smallest Tafel slope has shown 15Mn-CoP to be the most active catalyst. The HER measurements were also carried out in 0.5M H2SO4 (Figure 3d and 3e), 15Mn-CoP could reach a current density of 10 mA/cm2 when the applied overpotential was only 65 mV with a Tafel slope of 32 mV/dec, which was very close to the commercial Pt/C (30mV/dec). For other Mn-doped CoP, a clearer elucidation is illustrated in Figure 3f, and all catalysts with Mn atoms outperformed the pristine CoP noticeably. We have also tested the manganese phosphide in both 1M KOH and 0.5M H2SO4 and compared its performance with CoP and 15Mn-CoP in Figure S5. It could be concluded that manganese phosphide was not as active as pristine CoP and need overpotentials of 250 mV and 156 mV to reach a current density of 10 mA/cm2 in 1M KOH and 0.5M H2SO4, respectively. A comprehensive comparison between previously reported CoP and other doped CoP is in Table S2. Compared with CoP (63.5 mV/dec)42 and a Ce-doped CoP (70.3 mV/dec) nanoarray on Ti foil22, which needed overpotentials of 139 and 92 mV to reach a current density of 10 mA/cm2 in alkaline, the demanded


Page 12 of 25

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


overpotential of the 15Mn-CoP was only 100 mV with the smallest Tafel slope of only 53 mV/dec. It was noted that catalysts in our work could not exceed catalysts on substrates like nickel foam or carbon clothes, whose higher mass loading and larger geometry surface could lead to a larger current density. To further elucidate the intrinsic activity of CoP and Mn-doped CoP, EIS measurements and CV tests were carried in 1M KOH.43-44 The smallest semicircle radius of 15Mn-CoP in Figure 3g indicated the charge transfer resistance (Rct) was only 8 Ω. 15Mn-CoP took obvious advantage over pristine CoP, 5Mn-CoP, 10Mn-CoP, 15Mn-CoP, and 25Mn-CoP, which owned Rct of 32 Ω, 14 Ω,10 Ω, and 9 Ω, separately. The CV measurements (Figure S6) taken in alkaline suggested the ECSA value of 15Mn-CoP was 41 mF/cm2, which was much larger than the values of CoP (14m F/cm2), 5Mn-CoP (19 mF/cm2) and 10Mn-CoP (30 mF/cm2). Noted the larger ECSA values were obtained with the increment of Mn. Thus, the best HER performance of 15Mn-CoP attributed to the smallest Rct value and largest ECSA value. For HER, another importance figure to estimate the intrinsic property of different catalysts was the values of turn over frequency (TOF). Hence, a simplified method, which was reported to calculate the TOF of Ni2P, was adopted to evaluate the different TOFs of pristine CoP and Mn-doped CoP.18 The detailed calculation process was elaborated in Supporting information. Figure S7 demonstrate that the TOF of 15Mn-CoP was 1.5 times larger than pristine CoP and also larger than other Mn-doped CoP. The calculated values of TOF of 15Mn-CoP was 0.58 s-1, indicating the most efficient intrinsic activity of 15Mn-CoP among the obtained catalysts. The HER stability is another important issue for practical application. Hence, the durability measurements of 15Mn-CoP were both tested in acid and alkaline with a current density of 10 mA/cm2 for 20 hours (Figure 3h). After 20 hours, the increments



of overpotential were only 2.4mV and 1.1 mV in alkaline and acid, respectively. The LSV curves before and after long-term tests shown in Figure 3i indicated a small negative shift for 15Mn-CoP. The outstanding performance of 15Mn-CoP was comparable with the most active catalysts from the recent reports (Table S2). After stability measurement in 1M KOH for twenty hours, the area ratio of Mn3+/Mn2+ increased from 55% to 68%, which indicated the further oxidation process during the test. Figure S8 exhibited the area ratio of Co3+/Co2+ was 62.5%，which was only 1.5% larger than the fresh 15Mn-CoP. Moreover,the FESEM images of 15Mn-CoP after durability test in Figure S9 indicated that no obvious changes of the morphology were observed. And it suggested the prominent durability of 15Mn-CoP in both 1M KOH and 0.5M H2SO4 as efficient catalyst toward HER.

Figure 3. LSV curves and Tafel plots of catalysts in alkaline (a, b, c), LSV curves and Tafel plots of catalysts in acid (d, e, f), EIS tests (g), durability test of 15Mn-CoP in


Page 14 of 25

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


both acid and alkaline (h), and LSV measurement before (dash line) and after (solid line) durability test.

DFT calculation of Gibbs free energy of hydrogen adsorption It was widely accepted that Gibbs free energy (∆Gads,H) is a good description of the performance for different catalysts, and a neutral value of ∆Gads,H could accelerate the efficiency of proton reduction and desorption of the produced hydrogen. Thus, we applied DFT calculation to evaluate the ∆Gads,H on different sites of pristine CoP and Mn-doped CoP. The (101) surfaces of all catalysts were adopted in this work, which were thought to be the most thermal stable slabs.45 Figure 4a illustrates the active sites for hydrogen adsorption including the cobalt sites, the phosphorus sites, and the metal bridge sites. In Figure 4b, it was interesting to note that the smallest values of ∆Gads,H on the cobalt sites of Mn doped CoP was only -0.07 eV, which was favorable compared to pristine CoP (-0.157 eV). The values of ∆Gads,H for cobalt bridge and phosphorus sites on the Mn-doped CoP were -0.1 eV and -0.174 eV, which were much smaller than its cobalt sites. For pristine CoP, the values of ∆Gads,H were -0.148 eV and -0.18 eV on the cobalt bridge and the phosphorus sites, respectively. Thus, it could be concluded that the most active sites for both pristine CoP and Mn doped CoP were their cobalt sites. According to previous d-band theory, the strong interaction between the d-band of cobalt atom and the s-band of the hydrogen atom was the main factor to influence the values of ∆Gads,H. Thus, the DOS of Co atom with and without Mn atom were calculated (Figure 4c). The negative shift of the d-band for Co atom indicated that the strong interaction between cobalt and hydrogen (∆Gads,H= -0.157eV) could be weakened after doping Mn atoms (∆Gads,H= -0.07eV).27 It could be explained that the Mn atom with stronger electron negativity could provide part electrons to adjacent Co atoms as shown in XPS analysis. Clearly, Mn doping resulted in a more



thermal neutral ∆Gads,H and enhanced the catalytic performance notably. According to previous reports, the surface P sites played an importance role as the direct adsorption site and could provide partial electrons to the Co center.46 Our DFT analysis demonstrated that the value of ∆Gads,H of P sits was smaller enough to adsorb hydrogen atom. However, the Co sites and Co bridge sites, which are close to Mn atom, have more neutral∆Gads,H than P sites. It indicated that the adsorption of hydrogen atom on P sites was much easier but the releasing of produced hydrogen on the Co sites was more plausible. The hydrogen spillover effect could explain that the hydrogen ion adsorbed in the P sites obtained an electron to became an adsorbed-H atom.47 Then the neutral H atom could migrate to the neighboring Co centers due to the hydrogen spillover effect. Finally, the H atom combined a hydrogen ion and an electron to release the hydrogen gas.

Figure 4. The slab model of CoP and Mn-doped CoP (a), ∆Gads,H of CoP and Mn-doped CoP on different sites (b), and DOS analysis (c).

Conclusions


Page 16 of 25

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In this work, Mn is found to be an efficient and low-cost dopant for CoP, which could fortify the HER of CoP with a striking result. Even after 20 hours of durability test, the good performances of CoP were retained stable in both acid and alkaline. The outstanding performance of 15Mn-CoP only required overpotentials of 100 mV and 65 mV to drive a geometry current density of 10 mA/cm2 in alkaline and acid, which was comparable with the most active catalysts from the recent reports. The further investigation via DFT calculation suggested the smaller value of ∆Gads,H after Mn doping was mainly affected by the stronger electron negativity of Mn atoms, which could provide part electrons to the adjacent Co atoms and draw the ∆Gads,H closer to zero. Furthermore, this work could be broadened to inspire more studies for the catalysts design toward efficient HER, especially the metal phosphide. ASSOCIATED CONTENT Supporting Information More Characterizations of Mn-doped CoP. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author E-mail: [email protected]; [email protected]. Notes The authors declare no competing ﬁnancial interest. Acknowledgements References (1) Fang, M.; Dong, G.; Wei, R.; Ho, J. C., Hierarchical Nanostructures: Design for Sustainable Water Splitting. Adv. Energy. Mater. 2017, 7 (23), 1700559, DOI 10.1002/aenm.201700559. (2) Tan, C.; Luo, Z.; Chaturvedi, A.; Cai, Y.; Du, Y.; Gong, Y.; Huang, Y.; Lai, Z.; Zhang, X.; Zheng, L.; Qi, X.; Goh, M. H.; Wang, J.; Han, S.; Wu, X. J.; Gu, L.; Kloc, C.; Zhang, H., Preparation of



High-Percentage 1T-Phase Transition Metal Dichalcogenide Nanodots for Electrochemical Hydrogen Evolution. Adv. Mater. 2018, 30 (9), 1705509, DOI 10.1002/adma.201705509. (3) Shi, Y.; Zhang, B., Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem. Soc. Rev 2016, 45 (6), 1529-41, DOI 10.1039/c5cs00434a. (4) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H.; Snyder, J.; Li, D.; Herron, J.; Mavrikakis, M.; Chi, M.; More, K.; Li, Y.; Markovic, N.; Somorjai, G.; Yang, P.; Stamenkovic, V., Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343 (6177), 1339-1343, DOI 10.1126/science.1249061. (5) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S., Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6 (12), 8069-8097, DOI 10.1021/acscatal.6b02479. (6) Gao, M. R.; Zheng, Y. R.; Jiang, J.; Yu, S. H., Pyrite-Type Nanomaterials for Advanced Electrocatalysis. Accounts of chemical research 2017, 50 (9), 2194-2204, DOI 10.1021/acs.accounts.7b00187. (7) Yang, J.; Wang, K.; Zhu, J.; Zhang, C.; Liu, T., Self-Templated Growth of Vertically Aligned 2H-1T MoS2 for Efficient Electrocatalytic Hydrogen Evolution. ACS applied materials & interfaces 2016, 8 (46), 31702-31708, DOI 10.1021/acsami.6b11298. (8) Zhang, J.; Wang, T.; Liu, P.; Liu, S.; Dong, R.; Zhuang, X.; Chen, M.; Feng, X., Engineering water dissociation sites in MoS2 nanosheets for accelerated electrocatalytic hydrogen production. Energy & Environmental Science 2016, 9 (9), 2789-2793, DOI 10.1039/c6ee01786j.


Page 18 of 25

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Balogun, M.-S.; Huang, Y.; Qiu, W.; Yang, H.; Ji, H.; Tong, Y., Updates on the development of nanostructured transition metal nitrides for electrochemical energy storage and water splitting. Mater. Today 2017, 20 (8), 425-451, DOI 10.1016/j.mattod.2017.03.019. (10) Regmi, Y. N.; Waetzig, G. R.; Duffee, K. D.; Schmuecker, S. M.; Thode, J. M.; Leonard, B. M., Carbides of group IVA, VA and VIA transition metals as alternative HER and ORR catalysts and support materials. J. Mater. Chem. A 2015, 3 (18), 10085-10091, DOI 10.1039/c5ta01296a. (11) Wang, J.; Xu, F.; Jin, H.; Chen, Y.; Wang, Y., Non-Noble Metal-based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications. Adv. Mater. 2017, 29 (14), 1605838, DOI 10.1002/adma.201605838. (12) Xu, X.; Nosheen, F.; Wang, X., Ni-Decorated Molybdenum Carbide Hollow Structure Derived from Carbon-Coated Metal–Organic Framework for Electrocatalytic Hydrogen Evolution Reaction. Chem. Mater 2016, 28 (17), 6313-6320, DOI 10.1021/acs.chemmater.6b02586. (13) Ruan, Y.; Lv, L.; Li, Z.; Wang, C.; Jiang, J., Ni nanoparticles@Ni-Mo nitride nanorod arrays: a novel 3D-network hierarchical structure for high areal capacitance hybrid supercapacitors. Nanoscale 2017, 9 (45), 18032-18041, DOI 10.1039/c7nr05560a. (14) Zheng, J.; Chen, X.; Zhong, X.; Li, S.; Liu, T.; Zhuang, G.; Li, X.; Deng, S.; Mei, D.; Wang, J.-G., Hierarchical Porous NC@CuCo Nitride Nanosheet Networks: Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting and Selective Electrooxidation of Benzyl Alcohol. Adv. Funct. Mater.2017, 27 (46), 1704169, DOI 10.1002/adfm.201704169. (15) Callejas, J. F.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E., Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater 2016, 28 (17), 6017-6044, DOI 10.1021/acs.chemmater.6b02148.



(16) Xiao, P.; Chen, W.; Wang, X., A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution. Adv. Energy. Mater. 2015, 5 (24), 1500985, DOI 10.1002/aenm.201500985. (17) Hu, F.; Zhu, S.; Chen, S.; Li, Y.; Ma, L.; Wu, T.; Zhang, Y.; Wang, C.; Liu, C.; Yang, X.; Song, L.; Yang, X.; Xiong, Y., Amorphous Metallic NiFeP: A Conductive Bulk Material Achieving High Activity for Oxygen Evolution Reaction in Both Alkaline and Acidic Media. Adv. Mater. 2017, 29 (32), 1606570, DOI 10.1002/adma.201606570. (18) Pan, Y.; Yang, N.; Chen, Y.; Lin, Y.; Li, Y.; Liu, Y.; Liu, C., Nickel phosphide nanoparticles-nitrogen-doped graphene hybrid as an efficient catalyst for enhanced hydrogen evolution activity. J. Power Sources 2015, 297, 45-52, DOI 10.1016/j.jpowsour.2015.07.077. (19) Pramanik, M.; Tominaka, S.; Wang, Z. L.; Takei, T.; Yamauchi, Y., Mesoporous Semimetallic Conductors: Structural and Electronic Properties of Cobalt Phosphide Systems. Angew. Chem., Int. Ed. 2017, 56 (43), 13508-13512, DOI 10.1002/anie.201707878. (20) Xin, Y.; Kan, X.; Gan, L.-Y.; Zhang, Z., Heterogeneous Bimetallic Phosphide/Sulfide Nanocomposite for Efficient Solar-Energy-Driven Overall Water Splitting. ACS Nano. 2017, 11 (10), 10303-10312, DOI 10.1021/acsnano.7b05020. (21) Zhang, R.; Tang, C.; Kong, R.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X., Al-Doped CoP nanoarray: a durable water-splitting electrocatalyst with superhigh activity. Nanoscale 2017, 9 (14), 4793-4800, DOI 10.1039/c7nr00740j. (22) Gao, W.; Yan, M.; Cheung, H.-Y.; Xia, Z.; Zhou, X.; Qin, Y.; Wong, C.-Y.; Ho, J. C.; Chang, C.-R.; Qu, Y., Modulating electronic structure of CoP electrocatalysts towards enhanced hydrogen evolution by Ce chemical doping in both acidic and basic media. Nano Energy 2017, 38, 290-296, DOI 10.1016/j.nanoen.2017.06.002.


Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Tang, C.; Zhang, R.; Lu, W.; He, L.; Jiang, X.; Asiri, A. M.; Sun, X., Fe-Doped CoP Nanoarray: A Monolithic Multifunctional Catalyst for Highly Efficient Hydrogen Generation. Adv. Mater. 2017, 29 (2), 1602441, DOI 10.1002/adma.201602441. (24) Liu, T.; Liu, D.; Qu, F.; Wang, D.; Zhang, L.; Ge, R.; Hao, S.; Ma, Y.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X., Enhanced Electrocatalysis for Energy-Efficient Hydrogen Production over CoP Catalyst with Nonelectroactive Zn as a Promoter. Adv. Energy. Mater. 2017, 7 (15), 1700020, DOI 10.1002/aenm.201700020. (25) NORSKOV, J., Chemisorption on Metal-Surfaces. Rep. Prog. Phys. 1990, 53 (10), 1253-1295. (26) Efficient Electrocatalytic Water Oxidation by Using Amorphous Ni–Co Double Hydroxides Nanocages. DOI 10.1002/aenm.201401880. (27) NORSKOV., J. K., Electronic Factors In Catalysis. Prog. Surf. Sci. 1991, 38 (2), 103-144. (28) Du, Y.; Cheng, G.; Luo, W., Colloidal synthesis of urchin-like Fe doped NiSe2 for efficient oxygen evolution. Nanoscale 2017, 9 (20), 6821-6825, DOI 10.1039/c7nr01413a. (29) Zhang, K.; Park, M.; Zhou, L.; Lee, G.-H.; Li, W.; Kang, Y.-M.; Chen, J., Urchin-Like CoSe2as a High-Performance Anode Material for Sodium-Ion Batteries. Adv. Funct. Mater. 2016, 26 (37), 6728-6735, DOI 10.1002/adfm.201602608. (30) Zhang, C.; Huang, Y.; Yu, Y.; Zhang, J.; Zhuo, S.; Zhang, B., Sub-1.1 nm ultrathin porous CoP nanosheets with dominant reactive {200} facets: a high mass activity and efficient electrocatalyst for the hydrogen evolution reaction. Chem. Sci 2017, 8 (4), 2769-2775, DOI 10.1039/c6sc05687c. (31) Ruban., A.; Hammer., B.; Stoltze., P.; Skriver., H. L.; Nørskov., J. K., Surface electronic structure and reactivity of transition and noble metals J. Mol. Catal. A: Chem. 1997, 115, 421–429.



(32) Clark, S.; Segall, M.; Pickard, C.; Hasnip, P.; Probert, M.; Refson, K.; Payne, M., First principles methods using CASTEP. Z. Kristallogr. 2005, 220 (5-6), 567-570. (33) Grimme, S., Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem 2006, 27 (15), 1787-99, DOI 10.1002/jcc.20495. (34) Perdew, J.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77 (18), 3865-3868. (35) Pan, H., Metal dichalcogenides monolayers: novel catalysts for electrochemical hydrogen production. Sci. Rep. 2014, 4, 5348, DOI 10.1038/srep05348. (36) Hinnemann, B.; Moses, P.; Bonde, J.; Jorgensen, K.; Nielsen, J.; Horch, S.; Chorkendorff, I.; Norskøv, J., Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127 (15), 5308-5309, DOI 10.1021/ja0504690 (37) Sen, K. D.; Politzer, P., Approximate radii for singly negative ions of 3d, 4d, and 5d metal atoms. J. Chem. Phys. 1989, 91 (8), 5123-5124, DOI 10.1063/1.457607. (38) Song, W.; Ren, Z.; Chen, S. Y.; Meng, Y.; Biswas, S.; Nandi, P.; Elsen, H. A.; Gao, P. X.; Suib, S. L., Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction. ACS applied materials & interfaces 2016, 8 (32), 20802-13, DOI 10.1021/acsami.6b06103. (39) Hao., S.; Yang., L.; Liu., D.; Kong., R.; Du., G.; Asiri., A. M.; Yang., Y.; Sun., X., Integrating natural biomass electro-oxidation and hydrogen evolution: using a porous Fe-doped CoP nanosheet array as a bifunctional catalyst. Chem. Commun. (Cambridge, U.K.) 2017 53 (42), 5710-5713 DOI 10.1039/C7CC01680H.


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Zhu, G.; Yang, L.; Wang, W.; Ma, M.; Zhang, J.; Wen, H.; Zheng, D.; Yao, Y., Hierarchical three-dimensional manganese doped cobalt phosphide nanowire decorated nanosheet cluster arrays for high-performance electrochemical pseudocapacitor electrodes. Chem. Commun. (Cambridge, U.K.) 2018, 54 (66), 9234-9237, DOI 10.1039/c8cc02475h. (41) Wang, X.; Zhou, H.; Zhang, D.; Pi, M.; Feng, J.; Chen, S., Mn-doped NiP2 nanosheets as an efficient electrocatalyst for enhanced hydrogen evolution reaction at all pH values. J. Power Sources 2018, 387, 1-8, DOI 10.1016/j.jpowsour.2018.03.053. (42) Yang, X.; Lu, A.-Y.; Zhu, Y.; Hedhili, M. N.; Min, S.; Huang, K.-W.; Han, Y.; Li, L.-J., CoP nanosheet assembly grown on carbon cloth: A highly efficient electrocatalyst for hydrogen generation. Nano Energy 2015, 15, 634-641, DOI 10.1016/j.nanoen.2015.05.026. (43) Pan, Y.; Lin, Y.; Chen, Y.; Liu, Y.; Liu, C., Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution. J. Mater. Chem. A 2016, 4 (13), 4745-4754, DOI 10.1039/c6ta00575f. (44) Liu, Y.-R.; Shang, X.; Gao, W.-K.; Dong, B.; Li, X.; Li, X.-H.; Zhao, J.-C.; Chai, Y.-M.; Liu, Y.-Q.; Liu, C.-G., In situ sulfurized CoMoS/CoMoO4shell–core nanorods supported on N-doped reduced graphene oxide (NRGO) as efficient electrocatalyst for hydrogen evolution reaction. J. Mater. Chem. A 2017, 5 (6), 2885-2896, DOI 10.1039/c6ta10284k. (45) Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Nørskov, J. K.; Abild-Pedersen, F.; Jaramillo, T. F., Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends. Energy & Environmental Science 2015, 8 (10), 3022-3029, DOI 10.1039/c5ee02179k.



(46) Ha, D.-H.; Han, B.; Risch, M.; Giordano, L.; Yao, K. P. C.; Karayaylali, P.; Shao-Horn, Y., Activity and stability of cobalt phosphides for hydrogen evolution upon water splitting. Nano Energy 2016, 29, 37-45, DOI 10.1016/j.nanoen.2016.04.034. (47) Cheng, Y.; Lu, S.; Liao, F.; Liu, L.; Li, Y.; Shao, M., Rh-MoS2 Nanocomposite Catalysts with Pt-Like Activity for Hydrogen Evolution Reaction. Adv. Funct. Mater. 2017, 27 (23), 1700359, DOI 10.1002/adfm.201700359.


Page 24 of 25

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only

Theoretical calculation and detailed experiments revealed that Mn is an efficient dopant for clean and facile electrochemical hydrogen generation.


Urchin-like CoP with Controlled Manganese ... - ACS Publications

Recommend Documents